Recently, a great deal of attention has been directed to environmental concerns throughout the world, and therefore many countries have strictly strengthened regulations on use of environmental contaminants, e.g. heavy metals such as chromium (Cr), lead (Pb), cadmium (Cd) and mercury (Hg), polybrominated biphenyl (PBB), polybrominated diphenyl ether (PBDE) and the like. Specifically, typical examples of such environmental legislations include RoHS (Restriction of Hazardous Substances, effective from Jul. 1, 2006), WEEE (Waste from Electrical and Electronic Equipment, effective from Jul. 1, 2006), ELV (End-of-Life Vehicles, effective from Jan. 1, 2007) and REACH (Registration, Evaluation and Authorization of Chemicals), which were adopted by the European Union (EU). In order to cope with the trends of these restrictions on the use of such hazardous substances, there are required active countermeasures against new environmental management policies, such as development of environmentally friendly products, reduction of industrial wastes which might be generated from factories and plants, introduction of a green procurement policy and the like.
Conventionally, in order to impart corrosion resistance and coating adhesion to zinc- and zinc alloy-coated steel sheets, aluminum- and aluminum alloy-coated steel sheets, cold-rolled steel sheets and hot-rolled steel sheets which have been widely used as automotive materials, building materials and materials for household electric appliances, a surface treatment is generally conducted which involves coating of metal surfaces with a chromate film that is composed mainly of chromium as a principal component. Chromate treatments may be broadly divided into electrolytic chromating and application chromating. In this connection, the electrolytic chromating is usually performed by cathodic electrolysis of a metal sheet using a treatment solution which contains hexavalent chromium (Cr (VI)) as a main ingredient and also contains a variety of added anions such as sulfate, phosphate, borate, and halogens. On the other hand, application chromating involves preparation of a treatment solution by adding an inorganic colloid or inorganic anion to a solution with a portion of the hexavalent chromium portion reduced to trivalent chromium beforehand and immersing the metal sheet therein or spraying the metal sheet with the treatment solution.
Unfortunately, use of these methods requires various measures associated with working conditions and drainage treatment, due to toxicity of hexavalent chromium contained in the chromating solution. In addition, recycling and waste disposal of automobiles, household electric appliances and building materials, which use the thus surface-treated metal sheets, also suffer from problems of harmfulness to human and environmental pollution.
To this end, the world's steel makers have focused efforts on the development of chromium-free, surface-treated steel sheets which can meet a variety of required characteristics such as corrosion resistance and conductivity, even without containing hexavalent chromium. According to conventional arts, chromium-free, surface-treated steel sheets have been manufactured via a method involving primary coating of a metal salt film, which is primarily composed of phosphate as a principal component, on the surface of the steel sheet, followed by secondary coating of a resin film which is primarily composed of acrylic and urethane resins as a main component, or a method involving formation of resin films as the primary and secondary films.
Further, numerous methods have been hitherto proposed for the development of surface treating agents such as chromium-free, anticorrosive metal coating agents. For example, Japanese Patent Laid-open Publication No. Hei 11-29724 discloses a chromium-free, anti-rusting agent comprising a thiocarbonyl group-containing compound and phosphate ions, and further water-dispersible silica in a waterborne resin. This system exhibits corrosion resistance comparable to a level of corrosion resistance imparted by chromating treatment, but disadvantageously suffers from insufficient storage stability and also poor corrosion resistance performance of the thin film.
In addition, Japanese Patent Laid-open Publication No. Hei 10-60315 discloses a surface treating agent for steel structures, comprising a silane coupling agent having a specific functional group which is reactive with a water-based emulsion. However, corrosion resistance required in this Japanese Patent is for relatively mild test conditions such as in wet testing and is not comparable to that of the present invention which withstands severe conditions such as a salt spray test on the thin film, as performed in the present invention.
Additionally, as a coating method which is designed in consideration of conductivity while involving no use of conventional hexavalent chromium, methods of coating a metal sheet with polyaniline are disclosed in Japanese Patent Laid-open Publication Nos. Hei 8-92479 and Hei 8-500770. However, due to the presence of polyaniline having high rigidity and low adhesion between the metal and resin film, peeling of the resulting film can easily occur at polyaniline-metal interfaces and polyaniline-resin interfaces. Such a probability of peeling poses problems when it is desired to perform coating on the top part of the steel sheet, in order to impart designability, particularly anticorrosiveness and other functions. Films with low adhesion are generally known to have low corrosion resistance. In addition, use of polyaniline also results in poor workability such as production of large amounts of precipitates due to low solution stability, worsening of working conditions due to generation of poisonous odor, and the like. Furthermore, such polyaniline-based solution compositions require high-temperature drying and curing conditions.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a surface treatment composition for a steel sheet which is post-treated with a chromium-free composition, particularly an inorganic, aqueous, metal-surface treatment composition which is capable of securing corrosion resistance and electrical conductivity and is curable at a low temperature.
It is another object of the present invention to provide a steel sheet which is surface-treated by coating with the above-mentioned metal-surface treatment composition.